The present invention is concerned with ultrasonic generators for use in power ultrasonic applications, such as cleaning or welding.
Ultrasonics companies involved in power ultrasonic applications, such as cleaning and welding, have made considerable efforts over the past 10 to 20 years to develop electronic generators of higher efficiency than those readily available and usually base on conventional class B or class C circuits, as used in radio transmission--the area from which the earliest (vacuum tube) ultrasonic generators were drawn.
The disadvantages of class B and class C circuits are fully described in U.S. Pat. No. 3,648,188 (Ratcliff) to which reference is hereby directed. It is well recognised that class B enables a high output to be obtained from a transistor at the price of low efficiency whereas class C permits high efficiency at the cost of low output. Class Bx, described in U.S. Pat. No. 3,648,188, was developed with the aim of combining the advantages of both class B and C, with the disadvantages of neither. Further reference to class Bx amplifiers is made below.
The most common circuits in general use for power ultrasonics applications are based on the bridge principle in which the load is switched between high and low lines and requires pairs of transistors to effect the necessary switching action. The accompanying FIG. 1 shows diagrammatically a typical full-bridge circuit. The switches are controlled in pairs so that in a first condition S.sub.1 and S.sub.2 are open and S.sub.3 and S.sub.4 closed and in a second condition S.sub.3 and S.sub.4 are open and S.sub.1 and S.sub.2 are closed. In this manner the current through the load is repeatedly reversed at the necessary operating frequency.
FIG. 2 shows a known half-bridge arrangement utilising two switches S.sub.1 and S.sub.2. When S.sub.1 is closed and S.sub.2 open the load is charged up. When S.sub.1 opens and S.sub.2 is closed the load discharges. This cycle is repeated to provide the operating frequency.
The half-bridge of FIG. 2 operates by allowing one half sinusoid of the current to flow whilst S.sub.1 is closed, the period being determined by the LC resonant characteristic. At the end of this period, S.sub.1 must be made to open and S.sub.2 to close, resulting in a half-sinusoid of current in the reverse direction through the load circuit and so completing a full sinusoidal oscillation of the current. The voltage across the circuit is, however, not sinusoidal but is a square wave as a result of S.sub.1 and S.sub.2 forming a changeover switch, as illustrated in FIG. 3.
The configurations of FIGS. 1 and 2 have, however, inherent design problems. In particular they have the problem of dual conduction in which the switches may momentarily be closed together as a result of poor synchronisation of the necessary control signals or transistor switching speeds. In this event, the switches appear as a short-circuit across the D.C. supply.
However, with careful design, "zero voltage switching" can be achieved--this being a necessary condition for high-level circuit efficiency, although maximum operating frequency is limited by the dual conduction characteristic, when using commercially available power transistors, to about 100 kHz.
FIG. 4 shows the known class Bx circuit of U.S. Pat. No. 3,648,188 which operates to provide "zero crossing" of "zero voltage switching" in the manner described in that patent. A principal feature of the known circuit is the transformer T.sub.1 which has always been considered essential to the operation of this type of circuit. The transformer T.sub.1 supplies a d.c. path for the current through the transistor Q.sub.1 which has to be switched in order to generate the necessary power oscillation.
The presence of this transformer T.sub.1 is, however, disadvantageous in practice. It is a relatively massive and expensive component. Furthermore, in order to achieve the generator power output required, the transformer has to carry the full load current in its secondary winding and the load current plus d.c. supply current in its primary winding. Therefore, heavy conductors are needed for both windings to carry these large currents. Moreover, substantial magnetic core material is needed to handle the high flux levels created by the high frequency current. In order to reduce the heat generated, forced air cooling is required when high levels of ultrasonic power are to be generated with transformers of acceptable dimensions and cost. Although it is possible to avoid forced air cooling by loadsharing in which two transformers are used, whichever way is chosen the bulk and cost is high, especially where high reliability in high ambient temperatures is required.
A further disadvantage of the transformer T.sub.1 is the necessity for critical design. The inductance of the primary (on which the secondary is based) must be neither too low nor too high and so it has to be manufactured within close tolerances. (If the inductance is too low, too much current flows in the primary; if too high, it cannot supply sufficient energy under varying load conditions--typically the case with ultrasonic cleaning systems).
The commercial need is to develop circuitry that will lower the cost per watt of output power. Attention has previously been directed to the transistor circuitry because of, as mentioned above, the relatively low efficiency of class A and B and the relative ineffectiveness of class C in using the potential of the power transistor indicated by its maximum voltage and current characteristics. However, since the known circuit of FIG. 4 has theoretical transistor efficiencies of 100%, no further significant progress in circuit design seems likely in this direction.
The real problem, and the area where substantial inefficiency arises, is thus in the magnetic components and the principal objective of the present invention has been to improve the performance significantly in this area.